The use of catalytic converters in the exhaust gas systems of internal combustion engines is commonplace nowadays. For example, diesel engines in particular use oxidation catalytic converters which convert unburnt hydrocarbons (HC) and carbon monoxide (CO), and both diesel engines and gasoline engines use reduction catalytic converters which convert nitrogen oxides (NOx). Also known are three-way catalytic converters which combine the functions of the oxidation and reduction catalytic converters, and therefore catalytically convert all three components, and are used mainly in gasoline engines. In principle, all catalytic converters require a specific minimum temperature, the light-off or start-up temperature at which they convert 50% of the limited exhaust gas constituents. After an engine cold start this temperature is normally not yet reached, so that if no further steps are taken, the emissions, referred to as cold-start emissions, exit the exhaust gas system unconverted.
Present, and even more so, future exhaust gas legislations require cold-start emissions measured in standardized driving cycles to also be recorded for determining the total emissions of a vehicle. The desire for further reduction of emissions and the increasingly reduced legal exhaust gas limits also require the reduction of cold-start emissions and therefore reaching the operating temperature of the catalytic converter system sooner.
A common measure for reducing cold-start emissions entails placing relatively small-volume precatalytic converters close to the engine, also referred to as primary catalytic converters. Due to their limited volume and their placement close to the engine, precatalytic converters quickly reach their light-off temperature, then take over conversion of a large portion of the emissions until a main catalytic converter connected downstream has also reached its operating temperature.
An exhaust gas system is known from DE 100 21 421 A1 in which an exhaust gas turbine of an exhaust gas turbocharger is situated in a main line of the exhaust gas duct, and the turbine may be bypassed by a bypass line routed in parallel. Situated in the bypass line is a precatalytic converter configured as a three-way catalytic converter or as an HC adsorber. A controllable valve may alternately shut off the bypass line or the main line, in which intermediate settings may also be provided. After a cold start the entire exhaust gas flow is guided initially through the bypass line via the precatalytic converter. Once a main catalytic converter connected downstream has reached its start-up temperature, the valve is switched over and the exhaust gas flow is guided into the main line via the exhaust gas turbine.
US 2002/0132726 A1 describes an exhaust gas system which includes a main catalytic converter, downstream from which are two parallel exhaust gas lines which may be alternately closed and opened with the aid of a switchover valve. The two parallel exhaust gas lines are arranged concentrically with an internal main line surrounded concentrically by an auxiliary line in which a ring-shaped HC adsorber is situated. Branching off from the auxiliary line upstream from the HC adsorber is a return line which feeds hydrocarbons which are unburnt and desorbed by the adsorber to the internal combustion engine. After a cold start the internal main line is closed and the exhaust gas flow is guided over the HC adsorber, which adsorbs and/or chemisorbs the unburnt hydrocarbons HC and hydrocarbons not converted by the not yet operational main catalytic converter. Once the main catalytic converter has reached its operating temperature, thereby ensuring sufficient HC conversion, the exhaust gas flow is directed into the main line. As a result of the heating now taking place the hydrocarbons desorb from the HC adsorber and are delivered to the engine combustion via the return line.
A similar concept is known from U.S. Pat. No. 5,315,824, which also employs the concentric structure consisting of an externally situated HC adsorber with a cordierite coating and an internally situated main line, the main catalytic converter in this concept being connected downstream from the HC adsorber, however. In addition, the exhaust gas system includes a precatalytic converter which is situated close to the engine. In this design, the return line described in US 2002/0132726 A1 is omitted. Instead, the desorbed hydrocarbons are converted by the main catalytic converter connected downstream. After an engine cold start, the exhaust gas in this case is guided initially over the HC adsorber which stores hydrocarbons at temperatures up to about 90° C. By the time the HC adsorber, depending on the temperature, starts to desorb the hydrocarbons, the precatalytic converter has reached a temperature which permits conversion of hydrocarbons emitted from the engine. In this phase, the exhaust gas is guided through the main line in order to bypass the HC adsorber. Once the main catalytic converter connected downstream has reached its light-off temperature at least in some sectors, the exhaust gas is again guided over the HC adsorber in order to flush the hydrocarbons released from the HC adsorber into the main catalytic converter and to convert them. The design approach of guiding exhaust gas in parallel proposed herein is relatively complex.
A simpler and more cost-effective design for the parallel guidance of exhaust gas is known from JP 2008-133802 A (see FIG. 2 of the present application), in which the ring-shaped HC adsorber is supported on a concentric inner pipe which defines the main line and which projects upstream beyond the length of the HC adsorber. Situated in this projecting section of the inner pipe is a rotatable exhaust gas valve which may close or open the interior main line. In the closed position the exhaust gas is guided over the adsorber, whereas in the open position the major portion of the exhaust gas flow passes the interior main line due to the greater flow resistance of the adsorber. Moreover, the flow resistance of the auxiliary line is increased as a result of an aperture plate with defined through-holes which is situated downstream from the ring-shaped HC adsorber. However, the need for this aperture plate again means increased design complexity. Moreover, despite this measure, it is not possible to prevent entirely a leakage flow through the HC adsorber when the exhaust gas valve is opened.
Finally, common to all of the designs discussed above is the fact that the ring-shaped HC adsorber is mounted on a pipe which is situated concentrically inwardly, for which reason virtually the only carriers contemplatable for the HC adsorber are wound metal carriers. Fitting of ceramic carriers (monoliths), which due to their impact sensitivity require an embedding (canning) along both their exterior and interior circumferences, would be very costly. Nevertheless, the use of ceramic carriers is desirable inter alia because they may, for example, be provided with a particle filtering function.